This invention relates to a process for the manufacture of practically odourless benzoic acid by treatment with an inert gas at an elevated temperature.
Benzoic acid is manufactured on a large scale by oxidation of toluene with air (Ullmanns Enzyklopadie der technischen Chemie (Ullmann's Encyclopedia of Industrial Chemistry), 4th edition, volume 8, pages 366-370 (1974)). At the same time, by-products such as diphenyl and o-, m- and p-methyldiphenyl, hereafter conjointly designated as diphenyls, are formed, which, because of their strong odour, are objectionable even in small amounts, particularly if the benzoic acid is used and further processed, for example to give benzoic acid esters, which are used as plasticisers, dyestuff carriers, cosmetics and pharmaceuticals.
In the course of the customary working up of the reaction mixture obtained on oxidation of toluene with air, the diphenyls are not removed completely. Odourless benzoic acid can only be obtained by sublimation or recrystallisation with the aid of adsorbents, for example active charcoal, or by purification via sodium benzoate. Purification by distillation is technically virtually impossible, since diphenyl forms an azeotrope with benzoic acid, which only boils 4.degree. C. lower than benzoic acid itself. Because of the low solublity in water, recrystallisation from water is also not an industrially usable method. Even when the benzoic acid is processed further, the diphenyls are in many cases not removed automatically in the course of this processing, so that the end products also continue to exhibit the odour which, when they are used, is at times even more objectionable.
There is thus an urgent need for practically odourless benzoic acid, that is to say a benzoic acid of very low content of diphenyls, but hitherto no method achievable with simple technical means was available for satisfying this need.
In general benzoic acid is obtained, after manufacture and removal of unconverted starting material and of easily removable by-products by distillation or extraction, in the form of the melt, from which it is then obtained in the form of crystallised flakes, for example by using cooling rollers. There is also a great demand for benzoic acid in a finely divided form, which can only be obtained with difficulty by grinding, since benzoic acid shows an exceptional tendency to cake together after grinding.